According to an article by Eliza Strickland in yesterday's Discover Magazine:"Dogs have a sense of fairness, and get jealous and upset when several dogs perform a trick but only one is rewarded, a new study has found....
....While some owners may say that they’ve known about the deep emotional lives of their dogs for ages, the new experiments mark the first time a complicated emotion like jealousy has been observed in dogs in a controlled laboratory setting. “We are learning that dogs, horses, and perhaps many other species are far more emotionally complex than we ever realized,” [says] Paul Morris, a psychologist at the University of Portsmouth who studies animal emotions…. “They can suffer simple forms of many emotions we once thought only primates could experience” [Times Online].